Stable, high relaxivity MRI contrast agents. Contrast agents play an important role in clinical magnetic resonance imaging (MRI). About one third of scans employ a contrast agent and these agents are almost exclusively gadolinium (Gd) based T1 agents that provide positive image contrast. Recently, however, Gd-based agents have been implicated in nephrogenic systemic fibrosis (NSF), a debilitating disorder that affects patients with renal insufficiency. It is hypothesized that Gd dissociated from the contrast agent is a causative factor. For new contrast agents, whether they are non-specific extracellular tracers or targeted molecular imaging agents, the risk of Gd toxicity must be addressed. One approach to this problem is to design better Gd ligands that are more resistant to Gd release. A second approach is to increase the relaxivity (r1), i.e. T1 relaxtion efficacy, of the contrast agent. High relaxivity agents could be given at lower doses, reducing metal ion exposure to the patient. The parent application combines both approaches synergistically to identify new, stable, high relaxivity contrast agents. The goal of the parent application is to develop chemically stable contrast agents that provide high relaxivity over a broad range of magnetic fields and that can be readily "plugged in" to targeting groups for molecular imaging. Clusters of potent, stable gadolinium-based complexes that can be linked to a targeting group in a single step are synthesized and characterized with respect to relaxivity and stability in vitro (Aim 1). High relaxivity, stable clusters are assessed in rodent models for gadolinium dissociation and acute toxicity, and will be evaluated in a mouse glioma model for low dose imaging efficacy (Aim 2). As a proof of concept for this technology platform, we use the stable, high relaxivity clusters identified in Aim 1 and validated in Aim 2 to demonstrate that these reagents can be used to readily prepare peptide-targeted molecular imaging probes (Aim 3). We use these agents in an in vivo model to benchmark against current state-of-the-art targeted MRI contrast agents. In this competitive revision we add a fourth aim. An alternative approach to very high relaxivity agents is to prepare macromolecules containing 10s to 1000s of Gd chelates. It well established that while these macromolecular probes have dramatically improved sensitivity of detection, they are limited by incomplete elimination of Gd from the body because the molecules are too large to pass through the glomerular filter in the kidneys. We propose a strategy of constructing macromolecules containing 10's of stable Gd chelates using a hydrolytically unstable linker that allows controlled degradation of the macromolecule to smaller fragments that are completely eliminated from the body after the imaging study. We will prepare Gd-based macromolecules and evaluate 3 different hydrolytically unstable linkers. We will measure the kinetics of degradation in vitro and evaluate the MR angiographic imaging properties of these probes in mice and determine how fast and completely the Gd is eliminated from the body. The overall goals are a safe macromolecular contrast agent and hydrolytic degradation technology that is broadly applicable to MR molecular imaging applications. PUBLIC HEALTH RELEVANCE: This project involves developing magnetic resonance imaging (MRI) probe technology. We will produce MRI probes that may be safer than existing probes and which are better suited to take advantage of advances in MRI hardware. These probes may be useful in detecting coronary artery disease and identifying cancerous lesions.